Rocket nozzles and the like



July 7, 1964 Filed Feb. 10, 1961 Pawn z was/r7 N u a m m on w D. M. SCRUGGS ETAL ROCKET .NOZZLES AND THE LIKE 3 Sheets-Sheet 1 IOOO 6000 UTE-1 ENTORS Az g zy ifikaiy 760% j? M ATTOKA/E) United States Patent i 3,139,672 ROCKET NGZZLES AND THE 'LHiE David M. Scruggs and Robert H. Herron, South Bend, 11111., assignors to The Bendix Corporation, South Bend, Ind., a corporation of Delaware Filed Feb. 10, 1951, Sex. No. 88,481

5 Claims. (Cl.29-ll82.5)

The present invention relates to a method of increasing the strength and utility of metallic structures that are subjectedto high temperature environments; and more particularly to improved rocket nozzles and the like which are cooled by the newly discovered phenomena of microtranspiration.

An object of the present invention is the provision of a means for improving the strength of metals used at elevated temperatures.

Another object of the present invention is the provision of rocket nozzles and the like made from metal-ceramic composites which can withstand the erosive forces of high temperature gases better than does the metallic material from which the composite is made.

A more particular object of the present invention is the provision of structures which can withstand the erosive forces of high temperature gases for short periods under non-equilibrium conditions (as occurs in rocket nozzles, jetavators and the like) better than any other structures known heretofore particularly in neutral or reducing atmospheres.

The invention resides in certain constructions, and combinations and arrangements of materials; and further objects of the invention will become apparent to those skilled in the art to which the invention relates from the following description of certain preferred embodiments which are described with reference to the accompanying drawings forming a part of this specification, and in which:

FIGURE 1 is a graph of the relative weight of equal strength fibers of various metals at elevated temperatures;

FIGURE 2 is a cross-sectional view of a portion of the throat of a rocket nozzle after firing, which nozzle was made according to the teachings of the present invention;

FIGURE. 3 is a graph of the composition of the cermet material seen in FIGURE 2 as it varies away from the throat surface, and V FIGURE 4 is a graph plotting the temperatures at the front and back faces of test samples of a pure tungsten nozzle and a material of the present invention during exposure to flame having a temperature of 5500 F.

FIGURE 1 of the drawing shows how the strength of certain typical metals decreases rapidly at elevated temperatures above a temperature corresponding generally to the recrystallization temperature of the metal; and therefore points out the advantage of holding the temperature of these metals below their recrystallization temperature.

Applicants have discovered a new and improved sintered metallic-ceramic, composite material having unique properties among which is the property to undergo a phenomena. which applicants call micro-transpiration when the composite material is subjected to elevated temperatures. The material generally comprises a strong sintered matrix having small particles of a ceramic distributed uniformly throughout. This material when subjected to high temperature, develops, or already has, small microscopic pores at the grain boundaries which permits the escape of the ceramic, either in the liquid or vapor phase, to cool the structural matrix and maintains the matrix at a temperature below that of the environment. This phenomena applicants have called micro-transpiration. Obviously this phenomena is a physical one and can exist in any cermet material-provided the material must also be one which does not flux, comblue or otherwise attack the matrix metal, so as to reduce its melting temperature or affect the strength of the metal matrix in any way.

As a preferred embodiment of such a material, applicants have developed a tungsten-ceramic composite which undergoes less grain growth during sintering than does pure tungsten, and which sinters to a density above of the theoretical density at a relatively low sintering temperature. In order that the material can be sintered at relatively low temperatures, a very small amount of a sintering aid, such as nickel, iron or cobalt is used.

Iron and cobalt act as sintering aids when they are dis sintering aid which is used can greatly affect the sintered I structure of the tungsten which isobtained. Percentages down to approximately 0.10% of nickel for example are effective in promoting sintering of the tungsten. As the percentage of nickel used is increased up to approximately 3% by weight of the tungsten, the sintered matrix appears to become brittle due to the formation of a tungstennickel intermetallic compound. When the percentage of nickel is increased above approximately 3%, the ma trix begins to become ductile again due to the formation of a sintered nickel bond between the tungsten particles. For these reasons the amount of nickel used must-not exceed approximately 3% of the tungsten or the bond between the tungsten particles changes over to a nickel type of bond, and will melt at too low a temperature for the severe applications intended for the materials of the present invention. Cobalt and iron act generally in the same manner as nickel; so that the ratio by weight of the sintering aid to tungsten that is used, should therefore be from approximately 0.10% to approximately 3.0%, is preferably below 1.0%, and most preferably is about 0.5%. Where the material is sintered to a high density, the sintering aid at the grain boundaries melts to open up pores which communicate the ceramic with the surface of the cermet and permit the escape of. the melted or vaporized ceramic material. In those instances where great strength is not required, a loose matrix can be made without a sintering aid which will have existing open pores which communicate the ceramic to the exterior surface of the cermet material.

As previouslyindicated, the ceramic material which is used must not react in any way with the matrix metal to decrease its structural strength, and must melt at a temper ature below the melting point of the matrix so as to pro-. duce the desired cooling effect. In applicants preferred tungsten-ceramic composite, the metal oxides are the preferred ceramic material because of their generally low melting points, and because they do not form low melting point eutectics with the tungsten, as do most carbides and nitrides. The oxides provide another phenomena which is beneficialand so make them the preferred ceramic material. This other phenomena is produced by a slight oxidation of the tungsten to its oxides (believed to be predomi nately W0 which migrates to the fired surface of the material. Those ceramics which produce a low melting point eutectic or otherwise react with tungsten, obviously weaken the matrix and are therefore unsuitable for most applications.

Ceramic oxides in general have lower melting points and higher vapor pressures than do the nitrides and carbides. All of the ceramic oxides such as beryllia, alumina, thoria, silica, and magnesia can be melted and vaporized.

Patented J'uiy 7, 1964) Thoria, however, melts at too high a temperature to produce any appreciable cooling eflect on any metal. The use of zirconia is not recommended for use with tungsten because it reacts to form a zirconate with the tungsten. Silica melts at such a low temperature relative to tungsten that its cooling effect is dissipated before the temperature of the tungsten reaches a level where the cooling efi'e'ct is needed. The following is a chart showing the vyarious properties of these oxides and the various heats absorbed per one cubic inch of a 100% density material for various heat rises and transformation stages. The best information to date is that one molecule of alumina in ten dissociates at temperatures just above its melting point.

HEAT CAPACITIES IN IB.T.U./IN.

Solid Heat Sink 70 F. to

M P Y 216 160 198 78 250 v p 133 70 130 6 109 Liquid Heat Sink M t Heat of Vaporization 910 306 500 205 735 Heiat of Complete Dissociaon 1, 140 1, 020 700 570 845 Total Heat Required to Vaaorize Material Solid to as r 1,397 668 932 317 1,177 Heat of Reaction Form Where the use temperature is high enough to melt and vaporize beryllia, it will be seen that its use has the advantages of low weight and high total theoretical heat of vaporization; At use temperatures of approximately 5000 F., alumina gives substantially as good results as beryllia, even though alumina has a lower total theoretical heat of vaporization; because alumina has a lower melting point and, therefore, agreater percentage vaporizes. The second last line of the graph shows the additional heat which would be absorbed if the vapor which is produced dissociates into ionized metal and ionized oxygen atoms. The last line of the graph shows the heat absorbed by the reduction of the ceramic to its metal by the tungsten-the V tungsten being converted to W0 which sublimes.

In general, the cooling effect which is produced by the ceramic increases generally as the amount of ceramic is increased. With amounts of ceramic less than about of the composite or cermet by volume, the cooling eifect is not believed enough to be commercially significant. As the amount of ceramic is increased, the amount of metal decreases and thereby weakens the resulting structure. In general more than about 50% ceramic by volume causes the tungsten to become the discontinuous phase and so causes the ,whole cermet material to melt and be eaten away. The material of Patent 2,952,903 is such a material andwill not retain its shape when exposed to temperatures above the melting point of the ceramic, to approximately 30% by volume is generally the preferred range; with approximately to by volume being the most desirable range. While cermets of metals other than tugnsten can be made, cermets of tungsten have the greatest temperature resistance and are the preferred materials.

In addition to the above ingredients, other materials may be added for various reasons. In som'e'instances, a deoxidizersuch as titanium can be added to the tungsten cermet materials before sintering to help remove any free ogygen which might combine with the tungsten and retard its sintering. Titanium is not required, but tends to increase the density of the matrix that is achieved during sintering. Titanium also tends to alloy with the nickel to thereby aid in the distribution of the nickel throughout the matrix metal. Other metals which alloy with nickel can be used to extend the nickel. On a volume basis therefore the preferred material should comprise from about 50% to about tungsten, from about 10% to about 49% ceramic, and from about 0.5% to about 6.0% of a sintering aid. The material may also include from about Example I A nozzle throat liner was made by intimately blending the following weight percentages of powders:

% tungsten of 3 micron size /2 Nickel of minus 325 mesh 4.5% Beryllium oxide of minus 325 mesh The above mixture was hydrostatically pressed at 20,000 p.s.i. and sintered in hydrogen for 1 hour at 3200 F. The above weight percentages give the following percentages by volume 76.9% Tungsten 0.9% Nickel 22.2% Beryllia The material was of approximately 92% density; the nozzle had'a throat; and was tested for 41 seconds in a flame having a stagnation temperature of 6700 F.. The flame was produced by burning 61 pounds of Hercules aluminized solid fuel in a chamber at 600 p.s.i. average pressure. No loss in chamber pressure was experienced during the firing, and no measurable erosion took place. A similarly prepared nozzle having a A throat was tested for 60 secondsin a kerosene-oxygen flame of 5400. F. stagnation temperature. This flame was producedby burning kerosene at a rate of 45 lbs/hr. in oxygen at a rate of 1600 cubic feet per hour at 50 p.s.i. chamber pressure, and the gasesleaving thethroat had a velocity of -Mach 1.8 and temperature of 4800 F. The flame was highly oxidizing, but even so the nozzle only exhibited an erosion rate of 0.05 10- in./sec., while pure tungsten nozzles have an erosion rate of 5.0 1O- in./ sec.

Example 11 Another nozzle throat liner was prepared from a blend of the following weight percentages of powders:

93.9% Tungsten of 3 micron size 5.1% A1 0 of minus 325 mesh 0.5% Nickel of minus 325 mesh 0.5 Titanium of minus 200 mesh The above mixture was hydrostatically pressed at 20,- 000 p.s.i. and sintered for l hour in hydrogen at 3200 F. These percentages by weight convert to the following percentages by volume: I

76.9% Tungsten- 0.9% Nickel 1.6% Titanium 20.6% Alumina Example III Another nozzle throat liner was prepared from a blend of the following weight percentages of powder:

95.0% Tungsten of a nominal 3 micron size 4.0% Magnesium oxide of minus 325 mesh .5 Nickel of minus 325 mesh .5% Titanium of minus 200 mesh The above mixture was hydrostatically pressed at 20,000 p.s.i., and sintered for one hour in hydrogen at 2200 F. These percentages by weight convert to the following percentages by volume:

74.5% Tungsten .9% Nickel 1.6% Titanium 23.0% Magnesia This nozzle gave an erosion rate of 1.5 in./sec. in the oxygen-kerosene flame of Example I.

The temperatures of nozzle bodies cannot be recorded by means of thermocouples, and no other means has been devised so far for accurately recording the temperatures which are reached in the nozzle during firing. In nozzle structures that have been tested, only the throat liner was made of the cermet, and the liner in turn has often been encased in graphite. In these structures, it has been found that a considerable amount of tungsten carbide is produced on the outer surfaces of pure tungsten throat liners adjacent the graphite; while substantially no tungsten carbide is formed in the tungsten cermet liners of the above examples. Inasmuch as tungsten carbide forms rapidly above a temperature of 4500 F., it can be seen that the cermet nozzles are cooled by ablation to a temperature not appreciably exceeding 4500" F.

FIGURE 3 of the drawing is a graph showing the composition after firing of a nozzle prepared and tested in a manner similar to that given for Example I. It will be seen that a sizeable percentage of the ceramic has disappeared for approximately the first quarter inch adjacent the exposed surface, while the composition of the remaining inner portion of the liner has remained substantially un changed. The change in composition can visually be seen in the photograph shown in FIGURE 2 of the drawings. As evidence of vaporization, aluminum oxide wiskers have been found in the diverging portion of cermet nozzles containing aluminum oxide after firing. The tungsten cermet nozzles have also shown less sintering and melting eflect produced by the flame itself, than do the pure tungsten metal nozzles; so that it is very apparent that the tungsten cermet nozzles do not reach as high a body temperature during firing as do the pure tungsten or graphite nozzles. As seen in FIGURE 3, the ceramic has ablated without changing the shape of the strong tungsten metal matrix skeleton, or without appreciable erosion of its exposed surface. FIGURE 4 is a graph made from computer calculated data showing temperatures at the exposed and rear surfaces of pure tungsten sample, and those of a sample of a tungsten cermet constructed according to the teachings of the present invention.

It will be apparent to those skilled in the art that the objects heretofore enumerated as well as others have been accomplished, and that there has been provided a means of reducing the temperature of metals that are exposed to high temperatures. It will further be apparent that a sufficient teaching has been set forth above to enable any man skilled in the art to readily produce compositions of any metal and ceramic material which will produce the desired cooling effect.

While the invention has been described in considerable detail, I do not wish to be limited to the particular compositions above described; and it is my intention that the followings claims will cover all novel adaptations, modifications and arrangements of the invention which come within the practice of those skilled in the art to which the invention relates.

We claim:

1. A rocket flame confining and directing structure and the like in which the surface exposed to the flame is a metal-ceramic composite mixture consisting principally of a tungsten metal matrix having from about 0.10 percent to about 3.0 percent by weight of nickel based on the tungsten concentrated at the grain boundaries of the tungsten; said limit of about 3% by weight of nickel operative to insure a bond between tungsten grains resistant to melting at temperatures greater than the melting point of nickel; and said composite also including from about 10.0 volume percent to about 50.0 volume percent of beryllia dispersed throughout the tungsten at its grain boundaries.

2. A rocket flame confining and directing structure and the like in which the surface exposed to the flame is a metal-ceramic composite mixture consisting principally of a tungsten metal matrix having from about 0.10 percent to about 3.0 percent by weight of nickel based on the tungsten concentrated at the grain boundaries of the tungsten; said limit of about 3% by weight of nickel operative to insure a bond between tungsten grains resistant to melting at temperatures greater than the melting point of nickel; and said composite also including from about 10.0 volume percent to about 50.0 volume percent of alumina dispersed throughout the tungsten at its grain boundaries.

3. A rocket flame confining and directing structure and the like in which the surface exposed to the flame is a metal-ceramic composite mixture consisting principally of a tungsten metal matrix having from about 0.10 percent to about 3.0 percent by weight of nickel based on the tungsten concentrated at the grain boundaries of the tungsten; said limit of about 3% by weight of nickel operative to insure a bond between tungsten grains resistant to melting at temperatures greater than the: melting point of nickel; and said composite also including from about 10.0 volume percent to about 50.0 volume percent of magnesia dispersed throughout the tungsten at its grain boundaries.

4. A rocket flame confining and directing structure and the like in which the surface exposed to the flame is a. metal-ceramic composite mixture consisting essentially of: from approximately 74 to approximately 79 percent by volume of tungsten, approximately 1 percent by volume of nickel, said nickel content limited to insure a continuous tungsten phase resistant to melting at temperatures above the melting point of nickel, and from approximately 20 to approximately 25 percent by volume of beryllia.

5. A rocket flame confining and directing structure and the like in which the surface exposed to the flame is a metal-ceramic composite mixture consisting essentially of: from approximately 74 to approximately 79 percent by volume of tungsten, approximately 1 percent by volume of nickel, said nickel content limited to insure a continuous tungsten phase resistant to melting at temperatures above the melting point of nickel, and from approximately 20 to approximately 25 percent by volume of alumina.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A ROCKET FLAME CONFINING AND DIRECTING STRUCTURE AND THE LIKE IN WHICH THE SURFACE EXPOSED TO THE FLAME IS A METAL-CERAMIC COMPOSITE MIXTURE CONSISTING PRINCIPALLY OF A TUNGSTEN METAL MATRIX HAVING FROM ABOUT 0.10 PERCENT TO ABOUT 3.0 PERCENT BY WEIGHT OF NICKEL BASED ON THE TUNGSTEN CONCENTRATED AT THE GRAIN BOUNDARIES OF THE TUNGSTEN; SAID LIMIT OF ABOUT 3% BY WEIGBHT OF NICKEL OPERATIVE TO INSURE A BOND BETWEEN TUNGSTEN GRAINS RESISTANT TO MELTING AT TEMPERATURES GREATER THAN THE MELTING POINT OF NICKEL; AND SAID COMPOSITE ALSO INCLUDING FROM ABOUT 10.0 VOLUME PERCENT TO ABOUT 50.0 VOLUME PERCENT OF BERYLLIA DISPERSED THROUGHOUT THE TUNGSTEN AT ITS GRAIN BOUNDARIES. 